Earthed Cables and Unearthed Cables

Hi Yogesh Gupta,

In HT electrical distribution,the systm can be earthed or un earthed.

This same subject was discussed for a thread that came up in CR4 on 04/01/2008 . Please read my reply which is quoted below and please read all the other replies . It is always better to check the previous threads on the same subject before putting up a new thread.

"The selection of earthed/unearthed cable will depend on your system. If your distribution system is earthed then you have to use cable manufactured for earthed system. (which the manufacturer specifies). If the system is unearthed then you need to use cable manufactured for un earthed system.The unearthed system requires high insulation level"

"

For earthed and un earthed XLPE cables, the IS 7098 part2 1985 does not give any difference in specification. The insulation level for cable for unearthed system has to be more. In simple statement the manufacturers states that 11 KV earthed cable is suitable for use in 6.6 KV unearthed system.The process of manufacture of cable is same. The size of cable will depend on current rating and voltage level.

what IS specifies and I quote below:

" b) Voltage Grade (Uo/U)

1) Earthed System - 1.9/3.3 kV, 3.8/6.6 kV, 6.35/11 kV, 12.7/22 kV and 19/33 kV.

2) Unearthed System - 3.3/3.3 kV and 11/11 kV.

Note 1 - Cables of 6.35/11 kV grade (earthed system) are suitable for use on 6.6/6.6 kV (unearthed system) also "

Hope this will clear your doubts

Dear Abraham,

I have seen many discussion forums wherein this topic has been deliberated in detail. From your name I guess you are from India and perhaps from Kereal state. Assuming this to be valid, can you please tell me whether the 11 kV supply from KSEB or TNEB or from any other state electricity board should be considered as Earthed of UNearthed. I think that the EB sub-station might receiev power suply at 66 kV or 33kV, step it down to 11 kV for distribution. All purpose the step down Trfo at the EB S/S must be a Delta/ Delta configuration and there is no scope for earthing the neutral. This must be the case at almost all the distribution S/S. Then where does the need for a 11kV (earthed system) come into place? Can you kindly clarify this

Hi Mr. Gupta,

What Mr Abraham has said is very true. I am adding few details which I hope may clarify your questions.

(1) For cables to be used in solidly earthed system, the phase-to-armour insulation can be selected for U/root3 volts only. U0= U/root3

(2) If system neutral is resistance-earthed, then the phase-to-ground voltage of the two healthy phases rise up when an earthfault occurs on the third phase. And in healthy two phases the voltage rises to phase to phase value depending on the degree of effectiveness of earthing. Therefore, phase to armour insulation of cables should be U which has been representeted as U/U.

(3) If system is unearthed then as explained in (2) above the U/U will be the insulation level.

(4) IEC standards permit U0/U rating for cables used in high resistance earthed systems if Earth Fault relays clear the fault within 60 seconds - as the cables are tested for 2.2 to 2.8 times U values for 5/10 minutes.

I hope this may help you.

Can you tell which IEC allows U0/U rating for High resistance earthed system

dear yogesh,

1) in earthed cables the insulation level provided by the manufacturer between phase ( conductor ) and earth (armour) is 11kv/root3 (ie.phase voltage in 11kv system).

2) in unearthed system the insualtion level provided between phase (conductor) and earth (armour) is 11kv ( ie. line voltage in 11kv sysytem).

3) It is to be noted that insulation level between phase to phase ( conductors ) are 11kv in both earthed & unearthed systems, difference exists only in the insulation level between phase to earth in both systems.